1. Field of the Invention
This invention relates to a pneumatic radial tire having an improved durable life by improving fatigue properties of metal cords to be used in a carcass ply.
2. Description of the Prior Art
In a radial tire, the carcass ply is subjected to a static strain for retaining an internal pressure and a repeated strain during the running under loading, so that it is required to have strength for retaining the internal pressure and fatigue resistance durable to the repeated strain.
On the other hand, metal cords having a simplified twisting construction or a reduced filament number are usually desired in view of the improvement of metal cord productivity, and a layer construction of 3+9+1 is known as a most simplified twisting construction.
However, in order to apply the metal cord of such a layer construction to the carcass ply for radial tires, it is necessary to make the diameter of filament large for compensating the reduction of cord strength due to the reduction of filament number. When the filament diameter is made large to use in the carcass ply, surface strain due to bending increases and the fatigue resistance degrades, so that fatigue resistance is necessary to be improved to make the application of such metal cord to the radial tire possible.
As a factor of deteriorating fatigue properties of the carcass ply, there have hitherto been considered
(1) a fatigue of material due to repeated strain, PA1 (2) an abrasion fatigue between mutual contact portions of filaments (fretting phenomenon) and PA1 (3) a corrosion fatigue by water penetrating from exterior.
Particularly, it is seriously taken that the corrosion fatigue and fretting phenomenon considerably degrade the durable life of the tire.
As a countermeasure for solving the aforementioned drawbacks, it is disclosed in U.S. Pat. No. 4,258,543 and Japanese Patent laid open No. 56-43,008 that each filament constituting the cord is coated with rubber to reduce the abrasion due to mutual contact and to prevent the penetration of water. In order to attain the perfect rubber coating, it is necessary to leave a space between filaments. When such a cord is applied to the carcass ply, the arrangement of cords becomes non-uniform so that it is difficult to avoid premature breaking of a portion of the filaments constituting the cord due to uneven tension.
Furthermore, the coating of rubber cannot perfectly be achieved in the layer construction such as 3+9+1 or the like having the filament number larger than that of a single twisting construction.